A metal coated substrate, for example, zinc-nickel coated low alloy steel, often requires a post-treatment or a passivation, which may further improve physical properties, such as corrosion resistance, adhesion, property and the like. For example, an aircraft part or an automobile part which is exposed to various environments may be gradually deteriorated due to corrosion, thereby causing damages in the body or the surface. Therefore, high-performance post-treatment may be essential for proper operation or elongation of the life time of such parts.
In the related arts, the post-treatment may be applied as a coating by a standard method, such as spraying and immersing, or by an electrochemical process which includes either cathodic or anodic deposition processes. A protective layer may further be formed on the surface of the coated substrate. Alternatively, the surface of a metallic substrate may be modified by a combination of chemical and electrochemical processes.
Currently, chromate conversion coating has been widely used for passivation treatments on metallic substrates which include aluminum (Al) alloys, or zinc-nickel (Zn—Ni) alloy coated steel. However, due to its toxicity and carcinogen, the use of chromate is highly restricted and, thus, the need for alternative to chromates has increased.
In some efforts, alternative materials may be used instead of chromate as of corrosion inhibitive species. The alternative materials including metal cation, metal oxide, and non-metallic ion, have been used in passivation treatment to provide a protective layer with corrosion resistance; but a chemical reaction between ionic chemical species may occur in electrolyte or an aqueous solution before electrochemical treatment process, thereby producing undesired precipitations.
The description provided above as a related art of the present invention is just for helping understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.